Technology already exists relating to a rocker arm that is manufactured by subjecting a metal plate material to a press forming process (for example, see Japanese Patent No. 3207223 (pages 3 and 4, FIG. 6)). In this case, the side edges of a plate material that were punched into a predetermined shape are turned up to form a pair of retaining walls, a roller for contacting a cam is mounted between the facing retaining walls, and at one end, a guide groove for engaging with the upper edge of the valve stem of the internal combustion engine is formed by press forming the plate material in the plate thickness direction.
However, a depth of a predetermined amount or more is required for the guide groove so that the engagement with the valve stem does not disengage and the valve stem is not swayed by rocking of the rocker arm. In this case, when the plate material is pressed in the thickness direction thereof to form a guide groove as described above, and when the required depth of the guide groove is large, in some cases the drawing by the press becomes tight and causes the plate material to break at the time of formation. To avoid this situation, it is necessary to use a special material that has sufficient malleability as a plate material, and this leads to increased costs.